Optical Sensor System Including Series Connected Light Emitting Diodes

ABSTRACT

An optical sensor system having a light source comprising a plurality of series connected light emitting diodes (LEDs). The series connected LEDs may be switched at a predetermined frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from the following commonly owned U.S.Provisional Patent Applications: Ser. No. 61/165,171, Ser. No.61/165,181, Ser. No. 61/165,388, and Ser. No. 61/165,159, all of whichwere filed on Mar. 31, 2009.

This application is related to the following commonly-ownedapplications: U.S. Utility patent application Ser. No. __/___,___,entitled “CURRENT SOURCE TO DRIVE A LIGHT SOURCE IN AN OPTICAL SENSORSYSTEM”; U.S. Utility patent application Ser. No. __/___,___, entitled“DUAL VOLTAGE AND CURRENT CONTROL FEEDBACK LOOP FOR AN OPTICAL SENSORSYSTEM”; and U.S. Utility patent application Ser. No. __/___,___,entitled “HIGH VOLTAGE SUPPLY TO INCREASE RISE TIME OF CURRENT THROUGHLIGHT SOURCE IN AN OPTICAL SENSOR SYSTEM”; all filed on Jan. 5, 2010,and all of which are hereby incorporated by reference.

TECHNICAL FIELD

The present application relates to the sensors and, more particularly,to an optical sensor system including series connected light emittingdiodes.

BACKGROUND

Optical sensor systems may be used to locate and/or image an object bydetecting light reflected from the object. Such systems may include alight source that transmits light toward an object and a detector fordetecting portions of the transmitted light reflected by the object. Acharacteristic of the reflected light may be analyzed by the sensorsystem to determine the distance to an object and/or to generate anelectronic image of the object.

In one example, such a system may include a light source, such as one ormore light emitting diodes (LEDs), configured to transmit modulatedinfrared light (IR), i.e. IR light that is rapidly turned on and off.The detector may receive the reflected light and calculate the phaseshift imparted by reflection of the light back to the senor. The time offlight of the received light may be calculated from the phase shift anddistance to various points in the sensor field of view may be calculatedby multiplying the time of flight and the velocity of the signal in thetransmission medium. By providing an array of receiving pixels in thedetector, the distance signals associated with light received at eachpixel may be mapped to generate a three-dimensional electronic image ofthe field of view.

The manner of modulation of the light source in such systems is a factorin system performance. To achieve useful and accurate imaging, it isdesirable to modulate the light source at a high frequency, e.g. 40 MHz.In addition, it is desirable in such systems to modulate the lightsource with high efficiency and reliability, while maintainingreasonable cost of manufacture and a relatively small package size.

SUMMARY

In an embodiment, there is provided a light source circuit for anoptical sensor system. The light source circuit includes: a power supplyto provide a regulated direct current (DC) voltage output; a lightsource comprising a plurality of series connected light emitting diodes(LEDs); a current source coupled to the power supply and the lightsource to receive the regulated DC voltage output and to provide acurrent output; and a switch, the switch being configured to allow thecurrent output to through the plurality of series connected LEDs fromthe current source when the switch is closed and to prevent the currentoutput through the plurality of series connected LEDs when the switch isopen.

In a related embodiment, the circuit may further include a drive circuitto open and close the switch at a predetermined frequency. In a furtherrelated embodiment, the predetermined frequency may be substantiallyequal to 40 MHz.

In another related embodiment, the circuit may further include: a highvoltage supply circuit coupled to the plurality of series connected LEDsto provide a high voltage output; and a second switch, the second switchbeing configured to connect the high voltage output to the plurality ofseries connected LEDs from the high voltage supply when the secondswitch is closed and to disconnect the high voltage output from theplurality of series connected LEDs when the second switch is open. In afurther related embodiment, the circuit may further include a drivecircuit to open and close the second switch, the drive circuit beingconfigured to close the second switch at the start of an on time for theplurality of series connected LEDs to connect the high voltage output tothe plurality of series connected LEDs and to open the second switchduring a remainder of the on time of the plurality of series connectedLEDs to allow the current source to provide the current output to theplurality of series connected LEDs. In another further relatedembodiment, the circuit may further include a drive circuit to open andclose the switch and the second switch at a predetermined frequency. Ina further related embodiment, the predetermined frequency may besubstantially equal to 40 MHz.

In another further related embodiment, the circuit may further include adiode coupled between the current source and the plurality of seriesconnected LEDs, the diode being configured to conduct to provide thecurrent output to the plurality of series connected LEDs only when theswitch is closed and the second switch is open.

In yet another related embodiment, the current source may include: aninductor connected in series with a resistor; and a diode coupled inparallel with the inductor and resistor; and wherein the current sourceis configured to provided the current output through the inductor to theplurality of series connected LEDs when the switch is closed and divertcurrent through the inductor to the diode when the switch is open. In afurther related embodiment, the current source may include a currentmonitor coupled to the resistor and configured to provide the currentfeedback.

In another embodiment, there is provided an optical sensor system. Theoptical sensor system includes: a controller; a light source circuitcoupled to the controller to drive a light source including a pluralityof series connected light emitting diodes (LEDs) in response to controlsignals from the controller, the light source circuit including: a powersupply to provide a regulated direct current (DC) voltage output; acurrent source coupled to the power supply and the light source toreceive the regulated DC voltage output and to provide a current output;and a switch, the switch being configured to allow the current output tothrough the plurality of series connected LEDs from the current sourcewhen the switch is closed and to prevent the current output through theplurality of series connected LEDs when the switch is open; transmissionoptics to direct light from the light source toward an object; receiveroptics to receive light reflected from the object; and detector circuitsto convert the reflected light to one or more electrical signals;wherein the controller is configured to provide a data signal outputrepresentative of a distance to at least one point on the object inresponse to the one or more electrical signals.

In a related embodiment, the optical sensor system may further include adrive circuit to open and close the switch at a predetermined frequency.In a further related embodiment, the predetermined frequency may besubstantially equal to 40 MHz. In another related embodiment, theoptical sensor system may further include: a high voltage supply circuitcoupled to the plurality of series connected LEDs to provide a highvoltage output; and a second switch, the second switch being configuredto connect the high voltage output to the plurality of series connectedLEDs from the high voltage supply when the second switch is closed andto disconnect the high voltage output from the plurality of seriesconnected LEDs when the second switch is open. In a further relatedembodiment, the optical sensor system may further include a drivecircuit to open and close the second switch, the drive circuit beingconfigured to close the second switch at the start of an on time for theplurality of series connected LEDs to connect the high voltage output tothe plurality of series connected LEDs and to open the second switchduring a remainder of the on time of the plurality of series connectedLEDs to allow the current source to provide the current output to theplurality of series connected LEDs. In another further relatedembodiment, the optical sensor system may further include a drivecircuit to open and close the switch and the second switch at apredetermined frequency. In yet another further related embodiment, theoptical sensor system may further include a diode coupled between thecurrent source and the plurality of series connected LEDs, the diodebeing configured to conduct to provide the current output to theplurality of series connected LEDs only when the switch is closed andthe second switch is open.

In another related embodiment, the current source may include: aninductor connected in series with a resistor; and a diode coupled inparallel with the inductor and resistor; and wherein the current sourceis configured to provided the current output through the inductor to theplurality of series connected LEDs when the switch is closed and divertcurrent through the inductor to the diode when the switch is open. In afurther related embodiment, the current source may include a currentmonitor coupled to the resistor and configured to provide the currentfeedback.

In another embodiment, there is provided a method of providing lightoutput in an optical sensor system. The method includes: connecting aplurality of light emitting diodes (LEDs) in series; and switching acurrent through the plurality of LEDs at a predetermined frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages disclosedherein will be apparent from the following description of particularembodiments disclosed herein, as illustrated in the accompanyingdrawings in which like reference characters refer to the same partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating the principlesdisclosed herein.

FIG. 1 is a block diagram of an optical sensor system according toembodiments described herein.

FIG. 2 is a block diagram of optical sensor system light source circuitsaccording to embodiments described herein.

FIG. 3 is a block diagram of optical sensor system light source circuitsincluding a plurality of series connected LEDs according to embodimentsdescribed herein.

FIG. 3A is a timing diagram illustrating exemplary timing for closingswitches S1 and S2 according to embodiments described herein.

FIG. 4 is a circuit diagram of a high voltage supply according toembodiments described herein.

FIG. 5 is a circuit diagram of a switch S1 according to embodimentsdescribed herein.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of an optical sensor system 100according to embodiments disclosed herein. In general, the opticalsensor system 100 emits light 102, e.g. infrared (IR) light, that isreflected by an object 104, and receives the reflected light 106 toidentify the distance to the object 104 and/or to map an image of theobject 104. In some embodiments, for example, the system may beimplemented as a collision avoidance sensor, e.g. a back-up sensor, foran automotive vehicle. In a back-up sensor application, for example, thesystem provides a data output 108 indicating distance from the rear ofthe vehicle to an object 104 for assisting a driver of the vehicle inavoiding inadvertent contact with the object 104 when moving in reverse.Although systems and methods consistent with the present disclosure maybe described in connection with a particular application, those ofordinary skill in the art will recognize that a wide variety ofapplications are possible. For example, systems and methods consistentwith the present disclosure may be implemented in optical sensors forrange finding applications, or any application involving identificationand/or imaging of a target object.

Those of ordinary skilled in the art will recognize that the opticalsensor system 100 has been depicted in highly simplified form for easeof explanation. The optical sensor system 100 shown in FIG. 1 includescontroller/processing circuits 110, light source circuits 112,transmission optics 114, receiver optics 116 and detector circuits 118.The controller/processing circuits 110 may be known circuits forcontrolling modulation of a light source of the light source circuitsand for processing received data to generate an output data streamrepresentative of the distance from the sensor to the object and/or anelectronic image of the object. Controller/processing circuits 110 may,for example, be any of the depth sensor controller/processing circuitscommercially available from Canesta, Inc. of Sunnyvale, Calif.

The light source circuits 112 may include known circuitry for drivingthe light source in response to control outputs from thecontroller/processing circuits 110, and may include circuitry consistentwith the present disclosure. The transmission optics 114 may includeknown optical components for directing light output from the lightsource to provide a system field of view encompassing the object(s) ofinterest. The receiver optics 116 may include known optical componentsfor receiving light reflected from the object of interest and directingthe received light to the detector circuits 118. The detector circuits118 may include known light detectors, e.g. arranged in an array ofpixels, for converting the received light into electrical signalsprovided to the control/processing circuits 110. The detector circuits118 may, for example, be any of the detector circuits commerciallyavailable from Canesta, Inc. of Sunnyvale, Calif. The control processingcircuits 110 may calculate distance to various points on the object andwithin

FIG. 2 is a simplified block diagram of the light source circuits 112according to embodiments described herein. The light source circuits 112include a power supply 202, a current source 204 coupled to the outputof the power supply 202, a plurality of series connected plurality ofseries connected LEDs 206 coupled to the current source 204, a highvoltage supply circuit 208 coupled to the current source 204, and drivercircuits 210 for controlling switches S1 and S2 to turn the plurality ofseries connected LEDs 206 off and on at a predetermined frequency, i.e.modulate the plurality of series connected LEDs 206. Connecting theplurality of series connected LEDs 206 in series according toembodiments described herein avoids phase differences between LEDoutputs and provides cost efficiency. The term “coupled” as used hereinrefers to any connection, coupling, link or the like by which signalscarried by one system element are imparted to the “coupled” element.Such “coupled” devices, or signals and devices, are not necessarilydirectly connected to one another and may be separated by intermediatecomponents or devices that may manipulate or modify such signals. Thedriver circuits 210 may take one of any known configuration orconfiguration described herein.

The power supply 202 may take any known configuration for receiving aninput voltage from an input voltage source 212 and providing a regulateddirect current (DC) voltage output. The input voltage source 212 may be,as is shown in FIG. 2, a DC source, e.g. a vehicle battery, and thepower supply 202 may be, as is shown in FIG. 2, a known DC-DC converterfor converting the DC source voltage to a regulated DC voltage at theoutput of the power supply 202. Known DC-DC converters include, forexample, buck converters, boost converters, single ended primaryinductor converter (SEPIC), etc. In some embodiments, a SEPIC convertermay be used to allow a regulated DC output voltage that is greater than,less than, or equal to the input voltage. SEPIC converter and SEPICconverter controller configurations are well-known to those of ordinaryskill in the art. One SEPIC converter controller useful in connection asystem consistent with the present disclosure is commercially availablefrom Linear Technology Corporation, as model number LTC1871®. ThoughFIG. 2 shows a DC source voltage, those of ordinary skill in the artwill recognize that an alternating current (AC) input may alternativelybe used and the power supply 202 may then include a known AC-DCconverter for providing a regulated DC output voltage.

The current source 204 may provide a constant current to the pluralityof series connected LEDs 206 for energizing the plurality of seriesconnected LEDs 206 when the switch S1 is closed by the driver circuits210. The switch S1 is illustrated in diagrammatic form for ease ofexplanation, but may take the form of any of a variety of configurationsknown to those of ordinary skill in the art. For example, the switch S1may be a transistor configuration that conducts current under thecontrol of the driver circuit output.

The driver circuits 210 may be configured to open and close the switchS1 at a predetermined frequency under the control of control signals 214from the controller/processing circuits 110. In some embodiments, forexample, the driver circuits 210 may open and close the switch S1 at afrequency of about 40 MHz. The current source 204 may thus provide adriving current to the plurality of series connected LEDs 206 at thepredetermined frequency for modulating the plurality of series connectedLEDs 206, i.e. turning the the plurality of series connected LEDs 206 onand off.

The high voltage supply circuit 208 may be coupled to the plurality ofseries connected LEDs 206 through the switch S2. The switch S2 may beclosed by the driver circuits 210 under the control of control signalsfrom the controller/processing circuits 110 during the start of the “on”time for the plurality of series connected LEDs 206. A high voltage,i.e. higher than the output voltage of the power supply 202, may becoupled from the power supply 202 to the high voltage supply circuit208, e.g. by path 218, and the high voltage supply circuit 208 mayprovide a high voltage output V_(h) across the plurality of seriesconnected LEDs 206. In some embodiments, for example, the high voltageoutput V_(h) may be about 18V, whereas the regulated DC output of thepower supply 202 may be about 10V.

The high voltage supply circuit 208 may thus increase the voltage acrossthe plurality of series connected LEDs 206 to a higher voltage than canbe established by the current source 204 to overcome the parasiticinductance in the plurality of series connected LEDs 206 and decreasethe rise time of the current through the plurality of series connectedLEDs 206. After the start of the “on” time for the plurality of seriesconnected LEDs 206, the switch S2 may open to disconnect the highvoltage supply circuit 208 from the plurality of series connected LEDs206, and the switch S1 may be closed to allow the current source 204 todrive the plurality of series connected LEDs 206 through the rest of the“on” time. The switch S2 is illustrated in diagrammatic form for ease ofexplanation, but may take any of a variety of configurations known tothose of ordinary skill in the art. For example, the switch S2 may be atransistor configuration that conducts current under the control of theoutput of the driver circuits 210.

FIG. 3 illustrates a light source circuit including a plurality ofseries connected LEDs 206 a, e.g. infrared LEDs, according toembodiments described herein. Diodes D8, D9, D10, and D11 are coupledacross diodes D3, D4, D5, and D6, respectively, to take up any backvoltage across the diodes D3, D4, D5, and D6. Although FIG. 3 shows fourseries connected LEDs (that is, D3, D4, D5, and D6), it is to beunderstood that any number of LEDs may be connected in series to providea plurality of series connected LEDs.

In FIG. 3, the current source 204 a includes a resistor R1 in serieswith an inductor L1, and a diode D1 coupled in parallel across theseries combination of the resistor R1 and the inductor L1. A feedbackpath 302 to the power supply 202 is provided by a current monitor 304and a diode D2. As shown, the regulated DC output V_(s) of the powersupply 202 may be coupled to the input of the current source 204 a atthe resistor R1. The driver circuits 210 may open and close the switchS1 at a high frequency, e.g. 40 MHz. When the switch S1 is closed, acurrent I_(s) flows through the series combination of the resistor R1,the inductor L1, and to the plurality of series connected LEDs 206 a forenergizing the plurality of series connected LEDs 206 a. The inductor L1thus establishes a constant current source and limits the current I_(s)through the plurality of series connected LEDs 206 a when the switch S1is closed. When the switch S1 is open, however, no current flows throughthe plurality of series connected LEDs 206 a, and the current I_(L)through the inductor L1 is diverted through the diode D1 to maintaincurrent through the inductor L1.

As shown, the current monitor 304 may be coupled across the resistor R1for sensing the voltage drop across the resistor R1. The current monitor304 may take any configuration known to those of ordinary skill in theart. In some embodiments, for example, the current monitor 304 may beconfigured using a current shunt monitor available from TexasInstruments® under model number INA138. The current monitor 304 mayprovide a feedback output to the power supply 202, e.g. through thediode D2.

In response to the feedback from the current monitor 304 and during thetime when the switch S1 is closed, the power supply 202 may beconfigured to adjust the supply voltage V_(s) to a voltage that willallow the inductor L1 to recharge. In some embodiments, the feedbackpath 302 maybe coupled to a voltage feedback path of the power supply202 to provide a constant current control loop that takes control awayfrom the voltage control loop during the time when the switch S1 isclosed, i.e. “on” time for the plurality of series connected LEDs 206 a.A variety of configurations for providing an adjustable supply voltagein response to the current monitor feedback are well-known to those ofordinary skill in the art. In one embodiment, for example, the powersupply 202 may be configured as a known converter, e.g. a SEPICconverter, and a known converter controller, e.g. a SEPIC controllerconfigured to control the converter output in response to the currentmonitor feedback. A constant current may thus be established through theinductor L1 when the switch S1 is closed, i.e. when the diodes D3, D4,D5, and D6 are “on” and emitting light.

In FIG. 3, the high voltage supply circuit 208 is coupled to theplurality of series connected LEDs 206 a through the switch S2. Theswitch S2 may be closed by the driver circuits 210 under the control ofcontrol signals from the controller/processing circuits 118 during thestart of the “on” time for the plurality of series connected LEDs 206 a.When the voltage output of the high voltage supply circuit 208 iscoupled to the plurality of series connected LEDs 206 a, i.e. the switchS2 is closed, a diode D7 blocks the high voltage output of the highvoltage supply circuit 208 from the current source 204 a. After thestart of the “on” time for the plurality of series connected LEDs 206 a,the switch S2 may open to disconnect the high voltage supply circuit 208from the plurality of series connected LEDs 206 a. The diode D7 may thenconduct and the current source 204 a may drive the plurality of seriesconnected LEDs 206 a through the rest of the “on” time.

FIG. 3A is in exemplary timing diagram illustrating the timing of thesignal from the driving circuits for closing the switches S1 and S2. Asshown, the switch S2 may be closed at the start t_(s) of the “on” timefor a plurality of series connected LEDs to initially provide a highvoltage across the plurality of series connected LEDs to overcome anyparasitic inductance in the LEDs and thereby decrease the rise time ofthe current through the plurality of series connected LEDs. The switchS1 may then close, but the high voltage output V_(h) of the high voltagesupply may prevent the current source from sourcing current to theplurality of series connected LEDs while the switch S2 is still closed.The switch S2 may then open allowing the diode D7 to conduct and thecurrent source to drive the plurality of series connected LEDs duringthe remainder of the “on” time.

In FIG. 3A, driving current for the plurality of series connected LEDsis initially provided by the high voltage supply, e.g. by closing theswitch S2 as described above. The plurality of series connected LEDs mayexhibit significant parasitic inductance that limits a rise time of thecurrent source current therethrough. The high voltage output of the highvoltage supply overcomes the parasitic inductance of the plurality ofseries connected LEDs to allow a faster rise time of the current throughthe plurality of series connected LEDs than could be achieved by currentfrom the current source. When the switch S2 opens and the switch S1 isclosed, the diode D7 may conduct to allow the current source to drivethe plurality of series connected LEDs in the remainder of “on” time ofthe plurality of series connected LEDs. This configuration allows arelatively fast rise time of the current through the plurality of seriesconnected LEDs and a constant current from the current source throughthe plurality of series connected LEDs to allow switching/modulation ofthe output of the plurality of series connected LEDs at relatively highfrequency, e.g. 40 MHz. It is to be understood, however, that in someembodiments the high voltage supply is optional and may be omitted whenthe faster rise time provided thereby is not desired or necessary.

Those of ordinary skill in the art will recognize that a high voltagesupply may be provided in a variety of configurations. FIG. 4 is acircuit diagram of a high voltage supply circuit 208 a and a switch S2a. In FIG. 4, the switch S2 a is implemented using a first metal-oxidesemiconductor field-effect transistor (MOSFET) Q1 and a secondmetal-oxide semiconductor field-effect transistor (MOSFET) Q2 configuredand biased in cascode configuration. The first MOSFET Q1 is in a commonsource configuration and the second MOSFET Q2 is in a common gateconfiguration.

A high voltage input is coupled to the source of the first MOSFET Q1from a node in the power supply that has a higher voltage than theoutput voltage of the power supply. In some embodiments, for example,the drain of the power MOSFET in a SEPIC converter implementing a modelnumber LTC1871® SEPIC converter controller available from LinearTechnology Corporation may be coupled to the source of the first MOSFETQ1. The gate of the first MOSFET Q1 may be coupled to the drive circuit.The drive circuit may provide a square wave signal to the gate of thefirst MOSFET Q1 for causing the first MOSFET Q1 and the second MOSFET Q2to conduct periodically, i.e. to open and close the switch S2 a asdescribed above. When the first MOSFET Q1 and the second MOSFET Q2conduct, the high voltage across a resistor R2 and a capacitor C1 isprovided across the plurality of series connected LEDs.

As discussed above the switch S1 may be provided in a variety ofconfigurations known to those of ordinary skill in the art. FIG. 5 is acircuit diagram of a switch S1 a. In FIG. 5, the switch S1 a isimplemented using a first metal-oxide semiconductor field-effecttransistor (MOSFET) Q1 and a second metal-oxide semiconductorfield-effect transistor (MOSFET) Q2 configured and biased in cascodeconfiguration. The first MOSFET Q1 is in a common source configurationand the second MOSFET Q2 is in a common gate configuration and driven bythe first MOSFET Q1. This configuration allows a low impedance on thedrain of the first MOSFET Q1, thereby reducing the effects of Millercapacitance. Also, the voltage at the drain of the first MOSFET Q1 is nogreater than the gate voltage of the second MOSFET Q2. Accordingly, theswitching speed of the first MOSFET Q1 is independent of the voltage onthe drain of the second MOSFET Q2. In some embodiments, two switchesconfigured as illustrated in FIG. 5 may be used in parallel.

Unless otherwise stated, use of the word “substantially” may beconstrued to include a precise relationship, condition, arrangement,orientation, and/or other characteristic, and deviations thereof asunderstood by one of ordinary skill in the art, to the extent that suchdeviations do not materially affect the disclosed methods and systems.

Throughout the entirety of the present disclosure, use of the articles“a” or “an” to modify a noun may be understood to be used forconvenience and to include one, or more than one, of the modified noun,unless otherwise specifically stated.

Elements, components, modules, and/or parts thereof that are describedand/or otherwise portrayed through the figures to communicate with, beassociated with, and/or be based on, something else, may be understoodto so communicate, be associated with, and or be based on in a directand/or indirect manner, unless otherwise stipulated herein.

Although the methods and systems have been described relative to aspecific embodiment thereof, they are not so limited. Obviously manymodifications and variations may become apparent in light of the aboveteachings. Many additional changes in the details, materials, andarrangement of parts, herein described and illustrated, may be made bythose skilled in the art.

1. A light source circuit for an optical sensor system, the circuitcomprising: a power supply to provide a regulated direct current (DC)voltage output; a light source comprising a plurality of seriesconnected light emitting diodes (LEDs); a current source coupled to thepower supply and the light source to receive the regulated DC voltageoutput and to provide a current output; and a switch, the switch beingconfigured to allow the current output to through the plurality ofseries connected LEDs from the current source when the switch is closedand to prevent the current output through the plurality of seriesconnected LEDs when the switch is open.
 2. The light source circuitaccording to claim 1, the circuit further comprising a drive circuit toopen and close the switch at a predetermined frequency.
 3. The lightsource circuit according to claim 2, wherein the predetermined frequencyis substantially equal to 40 MHz.
 4. The light source circuit accordingto claim 1, the circuit further comprising: a high voltage supplycircuit coupled to the plurality of series connected LEDs to provide ahigh voltage output; and a second switch, the second switch beingconfigured to connect the high voltage output to the plurality of seriesconnected LEDs from the high voltage supply when the second switch isclosed and to disconnect the high voltage output from the plurality ofseries connected LEDs when the second switch is open.
 5. The lightsource circuit according to claim 4, the circuit further comprising adrive circuit to open and close the second switch, the drive circuitbeing configured to close the second switch at the start of an on timefor the plurality of series connected LEDs to connect the high voltageoutput to the plurality of series connected LEDs and to open the secondswitch during a remainder of the on time of the plurality of seriesconnected LEDs to allow the current source to provide the current outputto the plurality of series connected LEDs.
 6. The light source circuitaccording to claim 4, the circuit further comprising a drive circuit toopen and close the switch and the second switch at a predeterminedfrequency.
 7. The light source circuit according to claim 6, wherein thepredetermined frequency is substantially equal to 40 MHz.
 8. The lightsource circuit according to claim 4, the circuit further comprising adiode coupled between the current source and the plurality of seriesconnected LEDs, the diode being configured to conduct to provide thecurrent output to the plurality of series connected LEDs only when theswitch is closed and the second switch is open.
 9. The light sourcecircuit according to claim 1, wherein the current source comprises: aninductor connected in series with a resistor; and a diode coupled inparallel with the inductor and resistor; and wherein the current sourceis configured to provided the current output through the inductor to theplurality of series connected LEDs when the switch is closed and divertcurrent through the inductor to the diode when the switch is open. 10.The light source circuit according to claim 9, wherein the currentsource comprises a current monitor coupled to the resistor andconfigured to provide the current feedback.
 11. An optical sensor systemcomprising: a controller; a light source circuit coupled to thecontroller to drive a light source comprising a plurality of seriesconnected light emitting diodes (LEDs) in response to control signalsfrom the controller, the light source circuit comprising: a power supplyto provide a regulated direct current (DC) voltage output; a currentsource coupled to the power supply and the light source to receive theregulated DC voltage output and to provide a current output; and aswitch, the switch being configured to allow the current output tothrough the plurality of series connected LEDs from the current sourcewhen the switch is closed and to prevent the current output through theplurality of series connected LEDs when the switch is open; transmissionoptics to direct light from the light source toward an object; receiveroptics to receive light reflected from the object; and detector circuitsto convert the reflected light to one or more electrical signals;wherein the controller is configured to provide a data signal outputrepresentative of a distance to at least one point on the object inresponse to the one or more electrical signals.
 12. The optical sensorsystem according to claim 11, further comprising a drive circuit to openand close the switch at a predetermined frequency.
 13. The opticalsensor system according to claim 12, wherein the predetermined frequencyis substantially equal to 40 MHz.
 14. The optical sensor systemaccording to claim 11, further comprising: a high voltage supply circuitcoupled to the plurality of series connected LEDs to provide a highvoltage output; and a second switch, the second switch being configuredto connect the high voltage output to the plurality of series connectedLEDs from the high voltage supply when the second switch is closed andto disconnect the high voltage output from the plurality of seriesconnected LEDs when the second switch is open.
 15. The optical sensorsystem according to claim 14, further comprising a drive circuit to openand close the second switch, the drive circuit being configured to closethe second switch at the start of an on time for the plurality of seriesconnected LEDs to connect the high voltage output to the plurality ofseries connected LEDs and to open the second switch during a remainderof the on time of the plurality of series connected LEDs to allow thecurrent source to provide the current output to the plurality of seriesconnected LEDs.
 16. The optical sensor system according to claim 14,further comprising a drive circuit to open and close the switch and thesecond switch at a predetermined frequency.
 17. The optical sensorsystem according to claim 14, further comprising a diode coupled betweenthe current source and the plurality of series connected LEDs, the diodebeing configured to conduct to provide the current output to theplurality of series connected LEDs only when the switch is closed andthe second switch is open.
 18. The optical sensor system according toclaim 11, wherein the current source comprises: an inductor connected inseries with a resistor; and a diode coupled in parallel with theinductor and resistor; and wherein the current source is configured toprovided the current output through the inductor to the plurality ofseries connected LEDs when the switch is closed and divert currentthrough the inductor to the diode when the switch is open.
 19. Theoptical sensor system according to claim 18, wherein the current sourcecomprises a current monitor coupled to the resistor and configured toprovide the current feedback.
 20. A method of providing light output inan optical sensor system, the method comprising: connecting a pluralityof light emitting diodes (LEDs) in series; and switching a currentthrough the plurality of LEDs at a predetermined frequency.